not sticking to agreed on loan terms and overcharging me for the loan

got a home loan with standard bank last year with the agreement that if i moved my primary banking account to them the deal would be offered at prime lending rate. its over 9 months now and the loan interest rate is still charged at prime+0,43%. sent emails to query and an adjustment was made which only corrected for around R20 and yet their own online calculator indicates a drop in installement of around R400. now question is if they ever resolve this will they correct my installment for the past few months also to the correct agreed on interest rate. on a 20 year bond R400 over payment is a lot of money this bank will have schemed off me